1. Field of the Invention
This invention relates to a steam turbine plant, and more particularly to a steam turbine plant with at least three coaxially combined turbines and two reheaters.
2. Description of the Prior Art
Very high pressure turbine plants using very high pressure and high temperature steam are being developed. The steam pressure may be as high as 316 kg/cm.sup.2 g and the steam temperature, 593.degree. C. Such a turbine plant typically has very high pressure (VHP), high pressure (HP), intermediate pressure (IP) and low pressure (LP) turbines which are coaxially combined. It also has first and second reheaters (RHs). The first RH reheats steam discharged from the VHP turbine and supplies the reheated steam to the HP turbine. The second RH reheats steam discharged from the HP turbine and supplies the reheated steam to the IP turbine.
The steam discharged from the IP turbine is then supplied to the LP turbine. The steam discharged from the LP turbine is condensed in a condenser, and the condensed water is supplied to a boiler which generates VHP steam to be supplied to the VHP turbine.
The start-up procedure of such a turbine plant is very complicated and time-consuming, because abrupt heating-up of the turbine rotor and/or casing inlet portion or metal-steam temperature mismatching give thermal shock to the rotor and/or casing inlet portion and may cause damage to them. Furthermore, the fuel burning rate in the boiler cannot be smaller than a certain minimum value--typically 20 to 30% of the rated value in case of a coal burning boiler--to obtain a stable condition of the burner for the boiler.
A turbine bypass system is disclosed in a report entitled "An Integrated Turbine Bypass System" (Volume 43, Proceedings of the American Power Conference, 1981, Pages 130-141). Such a bypass system can be used to start up a turbine plant, for example, with a VHP turbine, an HP turbine, an IP-LP turbine and two reheaters. In such a turbine plant, the steam from the boiler is bypassed around the VHP turbine through a VHP bypass valve, the steam from the first reheater is bypassed around the HP turbine through an HP bypass valve, and the steam from the second reheater is bypassed around the IP-LP turbine through an LP bypass valve before the turbines rotate. Then a main steam control valve at the VHP turbine inlet, a first reheat steam control valve at the HP turbine inlet, a second reheat steam control valve at the IP turbine inlet are gradually opened to rotate the turbines. The VHP bypass valve, the HP bypass valve and the LP bypass valve are simultaneously closed to keep the pressure upstream of these bypass valves at certain values.
The steam remaining in the VHP turbine, however, can be excessively heated up because of gaseous friction which turns into thermal energy. The gaseous friction is especially large in a plant using very high pressure steam.
An appropriate flow rate of steam in the turbine can give a cooling effect to the turbine blades. It is, however, difficult to control all of the flow rates of all of the turbines and the turbine rotation, limiting the maximum metal temperature and minimizing metal-steam temperature mismatch.